The present invention relates to a device for connecting a subscriber line to a high frequency link and to a low frequency link. The invention is applicable to communications networks, in particular to the public switched telephone network (PSTN).
Such a network connects a subscriber to a switching center by means of a line. The subscriber has a telephone set which produces a low frequency signal and the subscriber also has a computer terminal which produces a high frequency signal, both of these two signals being conveyed together over the subscriber line.
In the switching center, the device is provided for connecting the subscriber line to a low frequency link and to a high frequency link which are respectively involved with conveying the low frequency signal and the high frequency signal. Typically, the high frequency link is a digital link with asymmetrical data rates, widely known as the acronym xe2x80x9cADSLxe2x80x9d for xe2x80x9casymmetrical digital subscriber linexe2x80x9d.
Separating the two signals does not raise any particular difficulty, except insofar as the low frequency signal can present interference which overlaps part of the spectrum of the high frequency signal. For example, the ringing signal for the telephone set is carried over the low frequency link and it is indeed a low frequency signal, however it is a pulsed signal and it therefore presents harmonics which constitute interference at high frequency. It is therefore appropriate to provide a lowpass filter function in the connection device to avoid injecting high frequency components from the low frequency link into the high frequency link.
This filtering function is conventionally performed by means of a specific module which requires numerous components.
An object of the present invention is thus to provide a connection device which considerably simplifies implementation of the filtering function.
According to the invention, the device for connecting a subscriber line to a high frequency link and to a low frequency link comprises a transformer disposed between said subscriber line and a signaling link, the transformer comprising a coupling coil designed to be fed by an encoder, the device also comprising a first switch for injecting either a ringing signal or a power supply signal into said signaling link in response respectively to a first state and a second state of a control signal; in addition, the device comprises a second switch for connecting or disconnecting the feed of said coupling coil in response respectively to the second and to the first states of said control signal.
Thus, the transformer performs a coupling function when the control signal is in its second state, and it provides the lowpass filtering function when the control signal is in its first state.
Advantageously, the device includes a lowpass filter disposed between said transformer and the subscriber line.
Thus, the lowpass filter comprises a capacitive element.
Preferably, said lowpass filter comprises a common mode stop coil disposed between said capacitive element and said transformer.
Similarly, it comprises a differential mode stop coil connected between said subscriber line and said capacitive element.
Furthermore, in a preferred embodiment, the device comprises a filter capacitor connected between said transformer and said signaling link.
Preferably, for device comprising a regulation loop having one end connected to the coupling coil, the other end of the regulation loop is connected to the output of the transformer, on the subscriber line side.